1. Field of the Invention
The present invention relates to a quantization control circuit regarding a bit reduction to reduce a bit rate of a digital video signal in a digital video signal recording apparatus.
2. Description of the Prior Art
A digital VTR which records a digital video signal onto a magnetic tape by, for example, a rotary head is known. Since an information amount of the digital video signal is large, a highly efficient coding to compress an amount of data which is transmitted is frequently used. Among various kinds of highly efficient coding methods, a DCT (Discrete Cosine Transform) is more and more put into practical use. The highly efficient coding using DCT is disclosed in the U.S. Pat. No. 5,006,931 which is proposed by the present applicant.
According to the DCT, an image of one frame is converted into a block structure of, for example, (4.times.4) blocks and each block is subjected to a cosine transforming process as a kind of orthogonal transformation. Thus, coefficient data of (4.times.4) are generated. Such coefficient data is subjected to a processing of a variable length coding of a run length code, Huffman code, or the like and, after that, it is recorded. In the recording mode, to make a data process on the reproducing side easy, a code signal as a coded output is inserted into a data area of a sync block of a predetermined length and there is performed a frame forming process to construct a sync block such that a sync signal and an ID signal have been added to a code signal.
In a digital VTR using a magnetic tape, a disk recording apparatus using a disk-shaped recording medium, or the like, video data of one field or one frame is generally recorded to a plurality of tracks. However, when a variable length output is formed as in the foregoing DCT, a data amount in a predetermined period of time fluctuates. Therefore, an equal length setting process (also called a buffering) to set a data amount in the predetermined period of time to a target value or less is necessary.
As an example of the equal length setting processes, there has been proposed an equal length setting process for controlling a data amount of a predetermined period of time (referred to as an equal length setting unit) that is shorter than one field or one frame and for setting the data amount to a target value or less even in a whole period of time of one field or one frame. The equal length setting process is a process for again quantizing coefficient data of an AC component generated by the DCT by proper quantization step and for suppressing a transmission data amount to a target value or less. Such quantization step are hereinafter called scaling factors. The scaling factor itself or an ID code to specify it is inserted into the transmission data together with the coded data.
In the quantization, it is necessary to decide the optimum scaling factor every equal length setting unit. Although a stronger data compression can be performed as a value of the scaling factor is large, a picture quality contrarily deteriorates. It is, therefore, necessary to minimize the value of the scaling factor in an allowable range of the bit rate. In the digital VTR, in processes such that an original image signal from, for example, a video camera is coded by the DCT and recorded onto a tape the reproduction data from the first generation tape is decoded and a first generation image is obtained, the same scaling factor can be used between the coding process and the decoding process by referring to the scaling factor or ID code indicative of the scaling factor in the recording/reproduction data.
However, in the dubbing mode such that the first generation image from a playback VTR is transmitted to a recording VTR through an interface and a second generation tape is formed by the recording VTR, it is necessary to use the same scaling factor as the scaling factor used when the first generation image is formed from the original image rather than the scaling factor is set to the minimum value. The same shall also apply to the case the first generation image is processed through a switcher and a special effect generating apparatus and the processed image is recorded. This is because even when the scaling factor is either larger or smaller than such a value, the picture quality deteriorates as compared with the first generation image.
Although the scaling factor can be individually transmitted in a home-use VTR or a communicating system, in a VTR for business use or broadcasting use which generally uses a digital interface such as CCIR601 or the like, it is difficult to transmit the scaling factor or ID code from a viewpoint of its format, so that it is necessary to determine the optimum scaling factor on the recording VTR side.
In the case where the scaling factor was determined on the recording VTR side as mentioned above, it has been found out that the relation between the scaling factor and the bit rate when the first generation image is formed from the original image differs from that in the subsequent multi-generation. FIG. 9 shows the results of experiment in the above case. That is, the relation between the scaling factor (axis of abscissa) and the bit rate (axis of ordinate) is plotted with regard to the case (shown by a solid line) of forming the first generation from the original image and the case (shown by a broken line) of forming the second generation from the first generation.
In the example of FIG. 9, when the target bit rate assumes 4.0 bits/pixel, the scaling factor when the first generation tape is formed from the original image is equal to 73. Subsequently, when the second generation image is formed by dubbing the first generation image which has been quantized by such a value, even in case of the scaling factor of 68, the bit rate can be set into a range. Therefore, the value (=73) when the first generation image is formed from the original image differs form the value (=68) when the second generation image is formed by dubbing. Namely, there is a problem such that the preceding scaling factor cannot be reproduced and the deterioration of the picture quality occurs upon dubbing only from a viewpoint of the restriction of the bit rate and, particularly, by repeating the dubbing process, the deterioration of the picture quality increases.